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Comet Struck Southern Nevada 370 Million Years Ago

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U.S. Department of the Interior
U.S. Geological Survey
Central Region Outreach Office
PO Box 25046, MS 150
Denver, CO 80225

Contact: Heidi Koehler
Phone: 303-236-5900x302
Fax: 303-236-5882

News Release: October 20, 1997
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Comet Struck Southern Nevada 370 Million Years Ago

An extraterrestrial object, theorized to be a comet at least one kilometer
in diameter, impacted what is now southern Nevada about 370 million years
ago. Evidence of the widespread damage caused by this collision was
announced October 20, 1997, at the annual meeting of the Geological
Society of America in Salt Lake City, Utah, by Charles A. Sandberg,
geologist emeritus of the U.S. Geological Survey, Denver, and his
collaborators, Jared R. Morrow, University of Colorado, and John E. Warme,
Colorado School of Mines.

"The impact, which is named the Alamo Impact after the town about 90 miles
north of Las Vegas, occurred offshore from a carbonate platform, very much
like the modern Australian Barrier Reef or the Bahamas Bank," said
Sandberg, who has been studying this area since 1991.

At the time of impact during the Devonian geologic Period, also known as
the age of sharks, an ancestral Pacific Ocean covered most of Nevada.
Shock waves from the impact and an ensuing initial 1,000-foot-high tsunami
wave caused extensive damage to the carbonate platform and coastline in a
semi-circular area 100 miles from north to south and 35 miles across. As the
carbonate platform collapsed, blocks hundreds to thousands of feet across
were torn from the seabed, twisted, and transported seaward. As tsunamis
of decreasing intensity reverberated back and forth across the ocean basin,
broken pieces of rock and ejecta from the impact were deposited over the
carbonate platform and high-water deposits were deposited in an bow-like
curve along the coastline to the east.

Three lines of evidence corroborate the extraterrestrial origin of the
triggering mechanism for the unusual phenomena observed in 13 mountain
ranges: shocked quartz grains, an iridium anomaly, and spherical carbonate
ejecta. Shocked quartz grains are sand grains pervasively shattered by the
force of an impact, iridium is a platinum-like element that is rare on
Earth, and carbonate spherules are formed from limestone fragments that
recrystallize within a superheated cloud. All three features have been
recorded in association with other impacts elsewhere on Earth, such as the
impacts that produced the extinction of the dinosaurs hundreds of millions
of years later. The Alamo occurred about three million years before and
may have led to an earlier extinction, one of the five great mass extinctions
recorded in Earth's history, in Late Devonian time.

Late-breaking field evidence suggests the existence of several channel
deposits filled with broken rock debris in the deeper ocean basin west of
the carbonate platform. Channel-fill deposits found at four localities show
that the area of impact-related phenomena occupies a circular area at least
120 miles in diameter. Whether these deposits represent debris that flowed
from the crater outward or slumped back into it has not yet been determined.
However, plotting a point midway between these sites and the possible
eastern crater rim at the west end of the Tempiute mountain range to the
east suggests location of the impact site, now buried beneath a valley or
much younger volcanic flows, north and west of the settlement of Rachel
and about 130 miles northwest of Las Vegas.

The U.S. Geological Survey provides the nation with reliable, impartial
information to describe and understand the Earth. This information is used
to minimize the loss of life and property from natural disasters and manage
water, biological, energy and other mineral resources in the wisest way
possible.